Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of sunlight (solar energy) to electricity. Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner.
Certain factors greatly affect the efficiency of CdTe PV modules in terms of cost and power generation capacity of the modules. For example, CdTe is relatively expensive and, thus, efficient utilization (i.e., minimal waste) of the material is a primary cost factor. In addition, the ability to process relatively large substrates on an economically sensible commercial scale is a crucial consideration.
An inherent problem of feeding granular CdTe material into a heated vapor deposition head under vacuum is that dose irregularities can result in non-uniformities in the subsequently formed thin film layer on the glass substrate. For example, dose quantities that are too large may cause the sublimation and resultant diffusion of the CdTe vapors onto the substrate to fluctuate to such an extent that film irregularities are produced.
Accordingly, there exists an ongoing need in the industry for improvement in the apparatus and method for feeding granular source material to a vapor deposition apparatus in the large-scale production of PV modules, particularly CdTe modules. The present invention relates to a feed system and that serves this purpose.